1. Field of the Invention
This invention relates to a compressor, or in particular, to a compressor having a thrust bearing.
2. Description of the Related Art
The conventional compressors of various types are used. The scroll compressor used for the refrigeration cycle is an example.
Generally, a scroll compressor includes a fixed scroll fixed to a housing and a movable scroll arranged in opposed relation to the fixed scroll and adapted to revolve on a rotary shaft with respect to the fixed scroll, so that a fluid is compressed by the fixed scroll and the movable scroll. The movable scroll receives the force in thrust direction due to the pressure difference between the pressure on the back of the movable scroll and the pressure of the fluid compressed. This force in thrust direction is supported by a thrust bearing.
The movable scroll orbits. In the case where the thrust bearing is used for the scroll compressor, therefore, the sliding speed is lower than in the case where the thrust bearing is used for a rotary device. For this reason, the oil film of the lubricating oil is difficult to form on the sliding surfaces, and a seizure is liable to occur.
In a compressor used in the refrigeration cycle in which carbon dioxide is the refrigerant, the pressure of the refrigerant compressed is so high that the force in thrust direction increases, and how to form an oil film on the sliding surfaces of the thrust bearing poses a more critical problem.
Japanese Patent Publication (A) No. 2001-115959, for example, proposes a scroll compressor including a thrust bearing having a sliding surface of a movable scroll and a fixed sliding surface, and a back pressure mechanism for reducing the load on the sliding surfaces by exerting pressure on the rear portion of the shaft of the movable scroll, wherein the sliding surfaces are covered with a diamond-like carbon layer.
This back pressure mechanism, however, is complicated in control operation and increases the cost. The absence of the back pressure mechanism to reduce the fabrication cost, on the other hand, may increase the wear of the thrust bearing and cause a seizure.